1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor, an electronic apparatus, and a driving method of an electronic apparatus capable of taking anti-blooming measures with a simple configuration.
2. Description of the Related Art
In the case of a digital camera, there are a global shutter method and a rolling shutter method as main electronic shutter methods of an image sensor. The global shutter method is a method of performing a simultaneous shutter operation on all pixels of a pixel array in which pixels are arrayed in a two-dimensional manner, and the rolling shutter method is a method of shifting pixels, in which a shutter operation is performed, in the unit of a row with time without performing the simultaneous shutter operation on all pixels.
Furthermore, in the case of a digital camera, there is an operation mode for image adjustment called a preview mode in which adjustment of a focus or a viewing angle, adjustment of exposure, and the like are performed before imaging a still image by performing reading on the entire pixel array of an image sensor.
In the preview mode, an image that the image sensor currently catches is displayed on a liquid crystal screen provided in a main body of a digital camera in order to make a user confirm the situation of the image, for example. However, since the number of pixels on the liquid crystal screen is smaller than the total pixel number of the image sensor, it is necessary to perform pixel number compression conversion for converting an image, which is obtained in all pixels of the image sensor, into an image with a smaller number of pixels corresponding to the number of pixels on the liquid crystal screen.
In this case, when a method of displaying an image on the liquid crystal screen by reading all pixels of the image sensor and performing pixel number compression conversion by digital signal processing is adopted, a current consumed is increased due to an image compression conversion operation, an operation of all pixels of the sensor, and the like. For this reason, a method of performing compression inside the image sensor by using a compression function of a compression circuit, which is provided in the sensor, in the V direction (vertical direction) and the H direction (horizontal direction) is generally adopted.
For example, in a CMOS (complementary metal oxide semiconductor) image sensor, a thinned-out image on which pixel number compression is executed is generated by performing pixel thinning out, in which discontinuous rows (lines of pixels) are selected and interposed rows are skipped, called V thinning out for the V direction.
Referring to FIG. 1, an operation of V thinning out in an image sensor using a rolling shutter method will be described. FIG. 1 is an example of a ½ thinning-out mode in which ½ thinning out is performed in the V direction.
In FIG. 1, a horizontal axis indicates a time using one horizontal scanning period (1 [H]), which is a time for which one row located in the horizontal direction of a pixel array section is scanned, as a unit, and a vertical axis indicates a row address that is an address of a pixel row located in the V direction. Moreover, in FIG. 1, an accumulation time (exposure time) of light (electric charges) is set to 5 [H].
In addition, in the following description, pixels of R (red), G (green), and B (blue) of an image sensor are assumed to be arrayed in the Bayer array.
Assuming that reading of electric charges is performed in (pixels of) a row corresponding to a row address n at predetermined time t [H], the accumulation time is 5 [H]Accordingly, a shutter operation, that is, an operation of sweeping out electric charges is performed at time (t−5) [H] that is 5 [H] earlier than time t [H]. In addition, in (pixels of) a row corresponding to the row address (n+1), a shutter operation is performed at time (t−4) [H] corresponding to reading at time (t+1) [H]. In addition, in the following description, performing a shutter operation is simply referred to as performing a shutter or also referred to as occurrence of a shutter.
At time (t+2) [H], rows corresponding to row addresses (n+2) and (n+3) are skipped and a row corresponding to a row address (n+4) is read. At time (t−3) [H], a shutter is performed in the row of the row address (n+4) corresponding to that described above. In addition, since a row corresponding to a row address (n+5) is read at time (t+3) [H], a shutter is performed in the row of the row address (n+5) corresponding to that described above at time (t−2) [H].
Following row addresses of read rows with a row of a row address n read at time t [H], a row read after the row of the row address n is a row of a row address (n+1) when moving a row address by 1 and a row read after the row of the row address (n+1) is a row of a row address (n+4) which has moved from the row address (n+1) by 3.
Similarly, a row read after the row of the row address (n+4) is a row of a row address (n+5) which has moved from the row address (n+4) by 1, and a row read after the row of the row address (n+5) is a row of a row address (n+8) which has moved from the row address (n+5) by 3.
That is, the rows read in the V direction are rows obtained by making a sequential movement with a movement amount of 1, 3, 1, 3, 1, 3, . . . . Accordingly, such V thinning-out operation is described as a V thinning-out operation of address addition amount (1, 3).
By performing the V thinning-out operation of the address addition amount (1, 3), two read rows and two skipped rows are alternately present in the V direction. The reason why two read rows and two skipped rows are alternately present in the V direction is because the image sensor is arrayed in the Bayer array.
That is, in the Bayer array, a GB row where pixels of G and B are alternately arranged and a GR row where pixels of G and R are alternately arranged are alternately arrayed in the V direction. Accordingly, since it is necessary to read electric charges with the GB row and the GR row adjacent to the GB row as a set, two read rows and two skipped rows are alternately provided.
In addition, it is sufficient not to continuously read GB rows or GR rows. That is, a GB row or a GR row adjacent to each other does not necessarily need to be read continuously.
In the V thinning-out operation of the address addition amount (1, 3) described with reference to FIG. 1, a row that is not read even once in one frame period is present. Specifically, the row that is not read even once in one frame period is a row corresponding to a row address (n+2) or a row corresponding to a row address (n+3), for example. In the case when such row that is not read even once in one frame period is present, saturated electric charges overflow from the row that is not read and leak to a read row. That is, a phenomenon called blooming occurs, and as a result, the quality of an image may be deteriorated. Here, one frame period is a period for which an image of one frame is read and is equal to 1 [H]×(the number of rows in the V direction). In a setting (15 fps) for reading 15 frames in a second, one frame period is about 63 msec.
Therefore, a read operation in which anti-blooming measures are taken is also performed in a known technique.
FIG. 2 is an example of a read operation in which anti-blooming measures are taken in the ½ thinning-out operation of the address addition amount (1, 3) described with reference to FIG. 1.
As the anti-blooming measures, a shutter is also performed on a row that is not read even once in one frame period. In the ½ thinning-out operation of the address addition amount (1, 3), an exposure regulation shutter and an electronic shutter (hereinafter, suitably referred to as an anti-blooming shutter) executed as anti-blooming measures are simultaneously performed on a pixel of a row address obtained by shifting a row address, in which an original electronic shutter (hereinafter, suitably referred to as an exposure regulation shutter) for regulating exposure is performed, by +2 rows as shown in FIG. 2. In FIG. 2, the exposure regulation shutter is indicated by the same double circle (⊙) as in FIG. 1, and the anti-blooming shutter is indicated by a black circle (●).
Thus, it is possible to prevent blooming by executing the anti-blooming shutter simultaneously with the exposure regulation shutter.
Next, an example of another V thinning-out operation mode in which anti-blooming measures are taken in a ¼ thinning-out mode of address addition amount (3, 5) will be described with reference to FIG. 3.
Since the address addition amount is (3, 5), rows read in the V direction are pixels of rows obtained by moving a row address with a movement amount of 3, 5, 3, 5, 3, 5, . . . .
That is, assuming that reading of electric charges is performed in a row corresponding to a row address n at predetermined time t [H], an exposure regulation shutter is executed at time (t−5) [H] that is 5 [H] earlier than time t [H]. A row read at next time (t+1) [H] is a row of a row address (n+3) which has moved by 3 from the row address n read before, and the exposure regulation shutter is executed at a row of the row address (n+3) at time (t−4) [H] that is 5 [H] earlier than time (t+1) [H].
Then, a row read at next time (t+2) [H] is a row of a row address (n+8) which has moved by 5 from the row address (n+3) read before, and the exposure regulation shutter is executed at a row of the row address (n+8) at time (t−3) [H] that is 5 [H] earlier than time (t+2) [H].
Therefore, at time (t−5) [H], the anti-blooming shutter is executed simultaneously with the exposure regulation shutter, which is executed in the row address n, at rows of the row addresses (n+1) and (n+2) which are rows skipped among rows until a row of a row address (n+3) in which the exposure regulation shutter is executed at next time (t−4) [H].
Similarly, at time (t−4) [H], the anti-blooming shutter is executed simultaneously with the exposure regulation shutter, which is executed in the row address (n+3), at rows of row addresses (n+4), (n+5), (n+6), and (n+7) which are rows skipped among rows until a row of a row address (n+8) in which the exposure regulation shutter is executed at next time (t−3) [H].
As described above, it is possible to take anti-blooming measures also in the ¼ thinning-out mode of the address addition amount (3, 5).
Moreover, the ¼ thinned-out image may also be generated in the case of the address addition amount (5, 3), (1, 7), or (7, 1) other than the address addition amount (3, 5) shown in FIG. 3.
Moreover, the ½ thinned-out image may also be generated in the case of the address addition amount (3, 1) other than the address addition amount (1, 3) shown in FIG. 2.
In addition, although not shown, in order to generate a ⅛ thinned-out image, a combination of address addition amounts includes eight kinds of (1, 15), (3, 13), (5, 11), (7, 9), (9, 7), (11, 5), (13, 3), and (15, 1).
Moreover, a ⅓ thinned-out image may be generated by a thinning-out operation of an address addition amount (3) that repeats a value 3 as an address addition amount.
In recent years, a thinned-out image is often used as an image when imaging a moving image as well as a preview mode. Accordingly, even in the case of an image after V thinning out, a request of a high-quality image is increasing.
In addition, the size of a liquid crystal screen is diversified in a digital camera for mobile phones, for example. For this reason, types of a V thinning-out operation mode tend to be diversified so that it is possible to meet various kinds of liquid crystal screen sizes with one image sensor.
In a known technique, in order to meet various kinds of thinning-out modes such that it is possible to meet various kinds of liquid crystal screen sizes with one image sensor, the combination of address addition amounts and the position of an anti-blooming shutter at that time are stored in a table. Then, in the case of generating a predetermined thinned-out image, required information is acquired from the table according to the generated thinned-out image and a V thinning-out operation is executed. Accordingly, since it is necessary to mount a large-capacity table in a logic circuit in order to meet a number of combinations, it has been difficult to reduce the gate size and the chip size.
Furthermore, in the case of performing short exposure under the situation where a large amount of light is incident, blooming occurs from a row adjacent to an object row of an exposure regulation shutter even if the anti-blooming shutter described above is executed. As a result, deterioration of the image quality has often occurred.
In addition, also in the case of performing all pixel reading, in which there is no skipped row and it is considered that the anti-blooming shutter is not needed, the blooming occurs from the row adjacent to the object row of the exposure regulation shutter. As a result, deterioration of the image quality has often occurred.
Referring to FIG. 4, occurrence of blooming from a row adjacent to an object row of an exposure regulation shutter in a case of performing all pixel reading at accumulation time of 3 [H] will be described.
In an example shown in FIG. 4, the exposure regulation shutter of a row address n read at time (t+3) [H] is executed at time t [H]. Adjacent rows of the row address n where the exposure regulation shutter is executed include two rows of row addresses (n−1) and (n+1). Since a sweeping operation is performed on the row of the row address (n−1) at previous time (t−1) [H], a photodiode is not saturated with electric charges in many cases. Accordingly, a possibility that blooming will occur is low.
On the other hand, in a photodiode corresponding to the row address (n+1), a sweeping operation previously performed is about one frame period before. Accordingly, since the photodiode is saturated with electric charges in many cases, the electric charges over flow easily. For this reason, as shown by an arrow in FIG. 4, blooming from the row address (n+1) to the row address n may occur immediately after the exposure regulation shutter of the row address n. Particularly in the case when the row address (n+1) corresponds to a GR row of the Bayer array and a component (red light) having a long wavelength is large in the amount of incident light, leakage from an R pixel of the GR row to a G pixel of a GB row is large and deterioration of the image quality, such as a false color, caused by a difference between a G pixel of the GR row and the G pixel of the GB row and blooming occur.
As a known technique related to anti-blooming measures, there is a technique of performing an electronic shutter (anti-blooming shutter) in a non-read region in order to avoid blooming from the non-read region (for example, refer to JP-A-2006-310932).
In addition, there is a method of alleviating blooming onto an adjacent pixel by continuously resetting floating diffusion in a power source and throwing away electric charges leaking to the floating diffusion into the power source or avoiding the blooming by adding a switch for resetting a photodiode of a pixel in a non-accumulation period (for example, refer to JP-A-2004-11590).